Expanded grid static mixer

ABSTRACT

A static mixer in the exhaust emission control system of an excess-air-operated combustion engine is formed of an expanded grid with a plurality of openings formed between crossbars. Using an expanded grid achieves both good mixing of the exhaust gas with a reducing agent in a short mixing path and properly aligns the exhaust flow.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of International ApplicationPCT/DE98/02780, with an international filing date of Sep. 18, 1998, nowabandoned.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a static mixer for placement ina flow channel and, more particularly, to an expanded grid static mixerfor improved mixing of exhaust gases and a reducing agent in an exhaustemission control system of an internal combustion engine.

[0004] 2. Description of Related Art

[0005] The use of a regulated or controlled diesel catalytic converterhas proven to be an advantageous technique to reduce pollutants,particularly nitrogen oxides, in the exhaust of an internal combustionengine. This technique is used in particular in internal combustionengines operated with excess air, such as diesel and lean-mix engines.This technique, based essentially on the method of selective catalyticreduction (SCR), brings the nitrogen oxides into contact with a reducingagent on a selective catalyst, which converts them into harmlessnitrogen and water.

[0006] An exhaust emission control system operating on this principle isdiscussed in a Siemens Aktiengesellschaft pamphlet entitled “SiNOxNitrogen Oxide Reduction for Stationary Diesel Engines,” Order No.A96001-U91-A232, Siemens AG, Bereich Energieerzeugung (KWU),Freyeslebenstraβe 1, 91058 Erlangen, Germany (1994). Such a system candrastically reduce the nitrogen oxide emissions from a diesel engine. Itmixes the nitrogen oxides in the exhaust gas with a reducing agent,usually ammonia obtained from urea by hydrolysis, in the exhaust pipe,and then introduces the mixture into a reactor space with a catalystdevice for catalytic conversion. A static mixer, such as that shown inDE 41 23 161 (U.S. Pat. No. 5,489,153), is used for mixing the reducingagent with the exhaust gas.

[0007] Known static mixers have proven generally satisfactory inhomogeneously mixing reducing agents with the exhaust gas in suchexhaust emission control systems. However, the exhaust pipeconfiguration in a vehicle must be adapted to the vehicle's structuraldesign, and in many applications the minimum mixing path required byknown mixers cannot be realized because of lack of space. In such acase, the flow conditions into the catalytic converter may beunfavorable, resulting in an uneven flow profile at the converter inlet.

SUMMARY OF THE INVENTION

[0008] It is an object of the present invention to avoid theshortcomings of prior art mixers in such situations.

[0009] The present invention relates to a static mixer for placement inthe flow channel of an exhaust emission control system, which seeks toprovide as short a mixing path as possible and align the flow directionof the exhaust gas in the flow channel.

[0010] In furtherance of the objects of the present invention, oneaspect of the invention involves an exhaust system for an internalcombustion engine having a catalytic converter for reducing pollutantsin the exhaust gases from the engine in the presence of a reducingagent, which exhaust system comprises an exhaust duct for carrying theexhaust gases from the engine to the catalytic converter, the exhaustduct having an injection location for the introduction of the reducingagent into the exhaust gases, and a static mixer disposed in the exhaustduct upstream of the catalytic converter and proximate to the injectionlocation, the static mixer comprising an expanded grid in the form of amesh having a plurality of openings formed between crossbars of saidexpanded grid.

[0011] In accordance with another aspect of the present invention, anexcess-air-operated internal combustion engine comprises a catalyticconverter for reducing pollutants in the exhaust gases from the enginein the presence of a reducing agent, an exhaust duct for carrying theexhaust gases from the engine to the catalytic converter, an injectiondevice in the exhaust duct for introducing the reducing agent into theexhaust gases upstream of the catalytic converter, and a static mixerdisposed in the exhaust duct upstream of the injection device, thestatic mixer comprising an expanded grid in the form of a mesh having aplurality of openings formed between crossbars of the expanded grid.

[0012] Another aspect of the present invention is a method of mixingexhaust gases of an internal combustion engine with a reducing agent,the method comprising providing a static mixer comprising a conventionalexpanded grid in the form of a mesh having a plurality of openingsformed between crossbars of the expanded grid, and placing the staticmixer in an exhaust duct of the engine proximate to an injectionlocation for the introduction of the reducing agent into the exhaustgases.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Exemplary embodiments of the invention are explained in moredetail with reference to four figures, in which:

[0014]FIG. 1 schematically depicts an exhaust emission control systemwith SCR catalytic reduction, having static mixers according to thepresent invention located in an exhaust pipe upstream and downstream ofa reducing agent injection location.

[0015]FIG. 2 is a detailed view of an embodiment of one of the staticmixers shown in FIG. 1, with a laminated construction in an inclinedorientation in the exhaust pipe.

[0016]FIG. 3 is ap perspective view of an embodiment of an expanded gridstatic mixer according to the present invention.

[0017]FIG. 4 is a section taken along the line IV-IV in FIG. 3.

[0018] In the drawings, the same components are given the same referencenumbers or letters in the different figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0019]FIG. 1 illustrates an exhaust gas A discharged from anexcess-air-operated internal combustion engine (not shown). The enginemay be a diesel engine with an exhaust turbocharger (not shown) thatdirects the exhaust gas A into a flow channel formed by an exhaust pipe2.

[0020] The exhaust pipe has therein a first static mixer assembly 18employing one or more expanded grids 4, discussed in more detail belowin connection with FIG. 3. Following in the direction of flow, aninjection device 6 for injecting a reducing agent R into the flow isdisposed downstream of the first static mixer, at a curved section 8 ofthe exhaust pipe 2. The reducing agent R may be ammonia (NH₃) inparticular an aqueous solution of urea. At a transitional point 10, theexhaust pipe 2 widens for receiving an exhaust-gas catalytic converter12. A second static mixer assembly 18′ is disposed downstream of theinjection device 6. The second mixer is located in the exhaust pipe 2just upstream of the transitional point 10, and therefore upstream ofthe catalytic converter 12. The second static mixer assembly 18′likewise employs one or more expanded grids 4.

[0021] The exhaust-gas catalytic converter 12 is shown as having threeSCR catalyst modules 14 arranged one behind the other. It serves in aconventional fashion to eliminate nitrogen oxides (NOx) in the exhaustgas A. An example of a suitable catalytic converter is shown inInternational Patent Application WO 97/01387, incorporated herein byreference as if set out in full. The cleaned exhaust gas A′ exits theexhaust pipe 2 via an outlet 16.

[0022]FIG. 2 illustrates in more detail the first static mixer assembly18 depicted in FIG. 1. The assembly 18 comprises plural static grids 4,in this case three grids 4 a, 4 b and 4 c. They are laminated, that is,they comprise a plurality of expanded grids 4 in series one behind theother, fitted into the exhaust pipe 2.

[0023] Each static mixer in FIG. 1 may be one expanded grid 4 or anassembly of plural, laminated expanded grids. In either case, thedistance X between the static mixers is preferably less than or equal tofive times the diameter D of the exhaust pipe 2.

[0024] In comparison with prior art mixers, use of an expanded grid as astatic mixer in an exhaust emission control system of this kind has beenfound to be an improvement both in providing more even distribution ofthe exhaust gas/reducing agent mixture and better alignment of the flowwithin the flow channel or exhaust pipe 2.

[0025] For instance, when the SINOx exhaust emission control system isused for a commercial vehicle in which the curved section 8 of theexhaust pipe 2 coming from the vehicle engine had to be installeddirectly ahead of the exhaust-gas catalytic converter 12 on account ofthe available installation space in the vehicle, it was possible to notean appreciable increase in the NOx conversion brought about by theinstallation of the expanded grid 4 as a mixer/flow director upstream ofthe curved section 8. It is believed that this increase in conversion ofNOx in comparison with known mixers can be attributed to the effectexerted by the respective expanded grid 4 on the direction of flowtherethrough. As is known, a pipe bend such as the curved section 8leads to the formation of an asymmetrical flow profile at the injectionpoint 19, determined by the position of the injection device 6, as wellas at the exhaust-gas catalytic converter 12.

[0026] As represented in FIG. 2, one or all of the expanded grids makingup a static mixer assembly according to the present invention may beinclined to the transverse direction 20 of the exhaust pipe 2 at anangle of inclination α to the longitudinal axis 21 of the pipe. In thiscase, the angle of inclination α is preferably between 0° and 45°.

[0027] An expanded grid used in the present invention may be made of anysuitable material, plastic or metal being typical. A metal grid of thetype suitable for use in the present invention is also sometimesreferred to as an expanded mesh. An expanded grid of this kind isdescribed, for example, in the company brochure “Strecken macht gröβer,Streckgitter” [“Expansion Increases Size, Expansion Grids”] of thecompany Ernst Sorst & Co., Hannover, Germany, and is commerciallyavailable in a variety of configurations. DE 32 08 634 C2 (U.S. Pat. No.4,567,630) discloses a manner of making a suitable expanded mesh sheet,in that instance used as a support for a catalytic substance in avehicle exhaust system. That U.S. patent and the Ernst Sorst & Co.brochure are incorporated herein by reference as if set out in full.

[0028] The individual expanded grids 4 a to 4 c used within a laminatedassembly 18 may also be of different types. For instance, various typesof the expanded grids represented in the company brochure mentionedabove, such as expanded grids with long crossbars and deformed orpleated expanded grids, may be combined within an exhaust emissioncontrol system.

[0029] As seen in FIGS. 3 and 4, the expanded-mesh grid 4 comprises amatrix of openings 24, which are bounded by crossbars 22 and throughwhich the exhaust gas A flows during operation of the exhaust emissioncontrol system. In this case, the effect of the direction of flow isdetermined essentially by the crossbar width b. The crossbar thicknessd, established by the thickness of the material used to make theexpanded mesh, and the length and width of the node points 26, formed byfour meeting crossbars 22, determine the effect of the mixing.Particularly good mixing and effective direction of flow are obtainedwith a ratio of the crossbar width b to the crossbar thickness d in therange between 0.5 and 5. The free cross-section, that is, the area ofall of the openings 24, should be between 25% and 85% of the total areaof the expanded grid 4 (and thus of the cross-sectional area πD²/4 ofthe exhaust pipe 2).

[0030] In accordance with the present invention, the static mixer alsofunctions as a flow director. During the operation of the exhaust systemand the exhaust emission control system, a reducing agent introducedinto the exhaust emission control system is mixed with the exhaust gason account of the mixing action generated by the expanded grid. At thesame time, the expanded grid aligns the flow at least approximatelyparallel to the longitudinal axis or axis of symmetry of the exhaustpipe 2. Aided by suitable arrangement of the expanded grid, unfavorableflow profiles within the exhaust pipe, induced in particular as a resultof given installation configurations required by vehicle structure, areeliminated, and the flow is made more uniform, all in a particularlysimple and effective way.

[0031] As noted above, the expanded grid typically fills the entirecross section of the exhaust pipe and about 25% to 85% of the expandedgrid is open to the flow. Depending on the application, the expandedgrid may be designed with a fine or coarse mesh and as an expanded gridwith long crossbars. The expanded grid may be installed normal to thelongitudinal axis of the exhaust pipe, or more preferably, at an angleof inclination up to 45°.

[0032] When the flow passes around the grid nodes or node points of theexpanded grid, vortexes and turbulence are generated to induce mixing.At the same time, the crossbars of the expanded grid align the flow ofthe exhaust gas, or of the exhaust gas/reducing agent mixture, accordingto the angle the crossbars present to the flow. With an increasing ratiobetween the crossbar width and crossbar thickness, an increasing effecton the flow direction is created. With a decreasing ratio, on the otherhand, increased mixing is obtained. As mentioned above, a particularlypreferred range of the ratio between crossbar width and crossbarthickness has been found to be between 0.5 and 5.

[0033] At least one of the expansion grids is advantageously arrangedupstream of the reducing agent injection point with respect to thedirection of flow of the exhaust gas. This is particularly advantageousif injection takes place within a bend of the exhaust pipe, since moreexhaust gas would otherwise occupy the outer region of the exhaust pipebend than in the inner region. The resultant unfavorable flow conditionsin the pipe bend normally lead to the exhaust gas being subjectedparticularly unevenly to the reducing agent when the latter is sprayedinto the exhaust pipe. That unfavorable flow profile is made moreuniform by the upstream expanded grid, so that homogeneous mixing of theexhaust gas with the reducing agent is provided even when injectiontakes place at or near a bend in the exhaust pipe. To make the flowprofile even more uniform as the exhaust-gas/reducing-agent mixtureenters the reaction space, a second expanded-grid static mixer isexpediently provided directly ahead of the catalytic converter.

[0034] If a number of expanded grids in a plurality of layers orlaminated assemblies are used, the alignment of the expanded grids inrelation to one another is performed initially with a view toparticularly good mixing of the exhaust gas with the reducing agent. Inthis case, the last layer downstream also serves for directing the flowaccording to the following pipe or channel routing. In the case of aplurality of layers, different expanded grids with different meshopenings, crossbar dimensions and/or node dimensions are preferablyused.

[0035] Use of an expanded grid as a static mixer in the flow channel ofan exhaust emission control system of an excess-air-operated combustionengine provides good mixing of the exhaust gas with a reducing agent,even over a particularly short mixing path. In addition, the flow of theexhaust gas and/or exhaust-gas/reducing-agent mixture is made moreuniform in a particularly simple way because the expanded grid alsoserves as a flow director.

[0036] Although preferred embodiments of the invention have beendepicted and described, it will be understood that various modificationsand changes can be made other than those specifically mentioned abovewithout departing from the spirit and scope of the invention, which isdefined solely by the claims that follow.

What is claimed is:
 1. An exhaust system for an internal combustionengine having a catalytic converter for reducing pollutants in theexhaust gases from the engine in the presence of a reducing agent, theexhaust system comprising: an exhaust duct for carrying the exhaustgases from the engine to the catalytic converter, said exhaust ducthaving an injection location for the introduction of the reducing agentinto the exhaust gases; and a static mixer disposed in said exhaust ductupstream of the catalytic converter and proximate to said injectionlocation, said static mixer comprising an expanded grid in the form of amesh having a plurality of openings formed between crossbars of saidexpanded grid.
 2. The exhaust system of claim 1, wherein said staticmixer is disposed upstream of said injection location.
 3. The exhaustsystem of claim 1, wherein said system includes a first static mixerupstream of said injection location and a second static mixer downstreamof said injection location.
 4. The exhaust system of claim 3, whereinsaid first static mixer and said second static mixer are spaced apart adistance X, said exhaust duct has a diameter D, and X is no greater thanfive times D.
 5. The exhaust system of claim 1, wherein said openingscomprise between 25% to 85% of a cross-sectional area of said exhaustduct.
 6. The exhaust system of claim 1, wherein said expanded grid isinclined at an angle a relative to a transverse direction normal to alongitudinal axis of said exhaust duct, and 0°≦α≦45°.
 7. The exhaustsystem of claim 1, wherein said crossbars have a width b and a thicknessd, and b/d is between 0.5 and
 5. 8. The exhaust system of claim 1,wherein said static mixer comprises a plurality of said expanded grids.9. The exhaust system of claim 8, wherein said plurality of saidexpanded grids comprise a laminated assembly.
 10. An excess-air-operatedinternal combustion engine comprising: a catalytic converter forreducing pollutants in the exhaust gases from the engine in the presenceof a reducing agent; an exhaust duct for carrying the exhaust gases fromthe engine to said catalytic converter; an injection device in saidexhaust duct for introducing the reducing agent into the exhaust gasesupstream of said catalytic converter; and a static mixer disposed insaid exhaust duct upstream of said injection device, said static mixercomprising an expanded grid in the form of a mesh having a plurality ofopenings formed between crossbars of said expanded grid.
 11. Theinternal combustion engine of claim 10, further comprising a secondstatic mixer disposed in said exhaust duct downstream of said injectiondevice, wherein said first-mentioned static mixer and said second staticmixer are spaced apart a distance X, said exhaust duct has a diameter D,and X is no greater than five times D.
 12. The internal combustionengine of claim 11, wherein each said static mixer comprises a laminatedassembly including a plurality of said expanded grids, in each of whichsaid openings comprise between 25% to 85% of a cross-sectional area ofsaid exhaust duct and said crossbars have a width b and a thickness d,and b/d is between 0.5 and
 5. 13. The internal combustion engine ofclaim 12, wherein each said laminated assembly is inclined at an angle arelative to a transverse direction normal to a longitudinal axis of saidexhaust duct, and 0°≦α≦45°.
 14. The internal combustion engine of claim10 wherein said injection device is disposed in a curved section of saidexhaust duct and said static mixer is disposed in a straight section ofsaid exhaust duct.
 15. A method of mixing exhaust gases of an internalcombustion engine with a reducing agent, the method comprising:providing a static mixer comprising an expanded grid in the form of amesh having a plurality of openings formed between crossbars of saidexpanded grid; placing said static mixer in an exhaust duct of theengine proximate to an injection location for the introduction of thereducing agent into the exhaust gases.
 16. The method of claim 15,wherein said static mixer is placed upstream of said injection location.17. The method of claim 16, further comprising placing a first staticmixer upstream of said injection location and a second static mixerdownstream of said injection location, wherein said first static mixerand said second static mixer are spaced apart a distance X, the exhaustduct has a diameter D, and X is no greater than five times D.
 18. Themethod of claim 17, wherein each said static mixer comprises a laminatedassembly including a plurality of said expanded grids, in each of whichsaid openings comprise between 25% to 85% of a cross-sectional area ofsaid exhaust duct and said crossbars have a width b and a thickness d,and b/d is between 0.5 and
 5. 19. The method of claim 18, wherein eachsaid laminated assembly is inclined at an angle a relative to atransverse direction normal to a longitudinal axis of said exhaust duct,and 0°≦α≦45°.